1. Field of the Invention
The present invention relates to an industrial polyester multifilamentary yarn of high modulus and low shrinkage, as a reinforcement for tires, and a dipped cord formed therefrom. More particularly, the present invention relates to a polyester multifilamentary yarn which retains superior dimensional stability and fatigue resistance even at high temperatures and a dipped cord formed therefrom. Also, the present invention is concerned with a method for producing such a polyester multifilamentary yarn and a dipped cord.
2. Description of the Prior Art
One of the typical functional uses which fibers have is to reinforce rubber composites, such as tires. Examples of the fibers useful as such a reinforcement include nylon, polyester, rayon, etc. Of them, polyester fibers contain benzene rings in their molecular structure, exhibiting a rigid property. Accordingly, tire cords produced from polyester yarns show high elastic modulus and few flat spots with superiority in fatigue resistance, creep resistance and endurance. By virtue of these high physical properties, polyester is extensively used as a reinforcement for rubber composites, especially tires.
In spite of these advantages, conventional polyester tire cords suffer from a significant disadvantage of reducing the side wall indentation of monoply radial tires. Also, industrial polyester yarns are required to improve in dimensional stability in order to replace the rayon fibers which have been applied for radial tires. In this regard, recent research has been directed to the development of polyester fibers which have high strength and elastic modulus in the same level as that of rayon fibers.
Techniques for increasing thermal stability in polyester fibers are Found in, for example, U.S. Pat. Nos. 4,101,525 and is 4,195,025 (both to Davis et al.) which disclose a polyester tire cord produced by drawing highly oriented undrawn yarn in a high-speed spinning process under a steaming condition to give highly oriented drawn yarn, especially multi-drawn yarn containing at least 85 mol % of polyethylene terephthalate, which ranges, in denier per filament, from 1 to 20 and, in work loss at 150.degree. C., from 0.004 to 0.02 lb.multidot.in, and dipping the multi-drawn yarn in a rubber solution.
Another prior art relating to a tire cord can be acquired from Japanese Pat. Laid-Open No. Sho. 61-12952 which discloses a process for producing a tire cord, comprising the steps of spinning a polyester having an intrinsic viscosity of 1.0, a diethylene glycol content of 1.0 mol %, a carboxyl group content of 10 eq/10.sup.6 g at a spinning speed of 2,000.about.2,500 m/min to obtain undrawn yarn, drawing the undrawn yarn at about 160.degree. C., thermally treating the yarn at 210.about.240.degree. C., and dipping the yarn. in an ordinary rubber solution. In this process, the temperature just below a spinning nozzle ranges from 100 to 450.degree. C. The tire cord thus produced is, however, poor in physical properties. For instance, the tire cord ranges, in absorption peak temperature in amorphous portions, from 148 to 154.degree. C. and, in dry shrinkage, from 3.3 to 5% with a tenacity of at least 7.0 g/d.
Focusing on high tenacity and low shrinkage, as introduced above, the research which was made on the development of the filamentary yarn for tire cords provided methods in which undrawn yarn with a high quantity of orientation and crystallinity is produced through spinning at a high stress and endowed with high tenacity and low shrinkage properties through drawing at a high draw ratio.
The yarns produced by the high-speed spinning or drawing according to the prior arts have improved fatigue resistance, but are problematic in that the molecular chain lengths in amorphous portions are non-uniform and extend. As a result, relaxed molecular chains coexist, giving rise to a great loss in tenacity. Thus, the yarns suffer from significant disadvantages of having poor drawability owing to a large difference in physical properties between inner and outer layers of the yarn and of exhibiting a great variation in physical properties owing to defects in their micro structure. Moreover, the yarns produced from a highly viscous polymer with an intrinsic viscosity of 1.0 or more show a limit of low shrinkage. Yarns which are drawn with a high orientation in advance of undergoing a tire cord conversion process have a definite two-phase structure of crystalline and amorphous portions. Where the highly oriented yarns are subjected to a thermal treatment by dipping in a rubber solution, deterioration is brought about in the crystalline portion with aggravation in the non-uniformity of the molecular chain, reading to a lowering of strength. As for polyester multi-filamentary yarn, it is highly apt to be damaged because it undergoes a series of after treatment processes. For example, at least two strands of the drawn yarn primarily obtained are subjecting to first and second twisting and formed into a fabric, after which the fabric is dipped in a rubber solution and incorporated into a rubber matrix of a tire, and during these processes, the yarn may be changed in physical properties and undergoes breaking of molecular chains.